This invention relates to polymeric compositions having excellent white light transmission that is substantially free of color over long light path lengths. Specifically, the invention relates to polymeric light pipe compositions having trace amounts of dyes that counteract the inherent absorption of red light to provide for the transmission of substantially colorless light when illuminated with a white light source. Furthermore, the invention also relates to light pipe compositions having trace amounts of dyes to provide for the transmission of light having a controlled color when illuminated with a white light source. The invention also relates to light pipe compositions having trace amounts of dyes that reduce the color of light when illuminated with an off-white light source.
Polymeric materials having excellent light-transmitting properties are known in the art. Such polymeric material is generally prepared using high glass transition temperature (xe2x80x9cTgxe2x80x9d) clear plastics such as polystyrene, polycarbonate, polymethyl methacrylate, polymethyl glutarimide, etc., and rubbery polymers such as crosslinked polyethylacrylate, polydimethylsiloxane, etc.
Applications requiring low color polymeric materials having excellent light-transmitting properties include light pipes for remote-source lighting systems, large plastic sheeting in which the edge color is minimized or controlled, plastic viewing windows or tanks for aquariums, and plastic block for windows.
One example of light transmitting polymeric materials are polymeric light pipes. Polymeric light pipes having excellent light-transmitting properties generally include a core material with a cladding material covering the core. Light pipes have been prepared using polymeric materials in the pipe core that have a low light absorption and low refractive index polymers for the pipe cladding.
One problem which exists for polymeric light pipes is that the length of the light pipe useful for transmission is limited. This is in contrast to inorganic glass optical fibers which have light transmission characteristics useful in fiber optic communication networks wherein digital light signals are transmitted over distances of tens of kilometers without significant attenuation. Polymers, on the other hand, have much higher absorption of light than do inorganic glasses, thus polymeric materials are limited to applications requiring transmission of light up to distances of tens of meters.
It is known that transparent polymeric materials have a light absorbance at peak maximum at a wavelength between 600 and 650 nm which arises from the vibrational overtones and harmonics of the carbon-hydrogen bonds. The wavelengths and assignments of the absorbance maxima of polymethyl methacrylate are 740 nm (xcexd5), 680 nm (xcexd5+xcex4), 630 nm (xcexd6), and 550 nm (xcexd7). (Takezawa, et al., Journal of Applied Polymer Physics, vol. 42, pp. 2811-2817, 1991). The absorbance at 630 nm (xcexd6), referred to in the art as the sixth harmonic of the carbon-hydrogen stretch causes white light passing through a meter or more of polymethyl methacrylate to appear green.
It is also known that the significant absorbances in polystyrene arising from carbon-hydrogen bond harmonics and overtones also occur at wavelengths between 600 nm and 650 nm. Thus, it is expected that all polymeric materials having carbon-hydrogen bonds will have an absorbance in the vicinity of 630 nm. Hence, it is expected that all low color polymeric materials having excellent light-transmitting properties and having carbon-hydrogen bonds will cause white light passing through a meter or more of the polymeric material to appear green. Although this is only a very slight color shift, this problem is particularly apparent in applications, such as light pipes for remote source lighting systems, wherein polymeric materials are required to transmit white light through a meter or more of the polymeric material.
Although glass has superior light transmission properties, rubbery polymeric fibers are much more flexible, workable, and lower in weight than glass optical fibers. These characteristics, together with their light transmission limitation of tens of meters, make polymeric light pipes useful for applications such as signs, instrumentation displays, medical devices, etc. Rubbery polymeric materials are especially useful for preparing large-core diameter (greater than about three millimeters) light pipes that are particularly useful in remote-source lighting applications where the large diameter allows the transmission of a large quantity of light from a single light source (illuminator) to multiple points up to tens of meters away.
Accordingly, it is desirable to provide relatively long, highly transparent, polymeric light pipes wherein no such color change occurs as the light path through the polymeric material is lengthened. These light pipes could provide light with a color that is independent of length. In end light applications, light pipes of different lengths can be used since all lengths will provide illumination of the same color. In side light applications, the color of the light will not change along the length.
In certain applications, such as remote source lighting, it is also desirable to provide relatively long, highly transparent, polymeric light pipes in which the color is controlled. Colored light is useful for lighting merchandise in display cases whereby the merchandise appears to be more appealing with light having a slight hue rather than pure white light. For example, light having a bluish hue is useful for illuminating yellowish articles, such as jewelry and the like, to make these articles appear desirably whiter. On the other end of the spectrum, light having a reddish hue provides a xe2x80x9cwarmerxe2x80x9d appearance to furniture and household goods. Also, some light sources produce light that is off white or yellow. In these cases it is sometimes desirable to reduce the color of the light, or make the transmitted light whiter.
One attempt to overcome the problems associated with the absorbance of light arising from the carbon-hydrogen bonds of the polymeric cores in optical fibers is disclosed in U.S. Pat. No. RE 31,868 to Beasley, et al. Beasley discloses several polymeric materials that have substantially diminished absorption arising from carbon-hydrogen covalent bonds, which as a result, have remarkably high transmission of light in the visible spectrum. The suitable polymeric materials include acrylates and methacrylates containing deuterium wherein the carbon-hydrogen covalent bonds have been replaced by carbon-deuterium covalent bonds. Optical fiber cores disclosed by Beasley are copolymers that contain a percentage of deuterium containing methyl methacrylate or deuterium containing polymethyl methacrylate polymer itself. As a result, polymers disclosed by Beasley are not expected to cause white light to turn green over relatively long distances compared to light pipe cores having hydrogen containing polymethyl methacrylate. Unfortunately, compared to commercially-available hydrogen containing monomers, deuterium containing monomers are only available in small research quantities, thereby making large-core optical fibers having deuterium containing polymers impracticable and prohibitively expensive for applications such as remote source lighting.
One attempt to overcome the problem of providing colored light emanating from polymeric light pipes is disclosed in U.S. Pat. No. 5,579,429 to Naum. Naum discloses a large core optical fiber with a fluorescent dye that provides a monochromatic neon-like side emission. The core is a methyl methacrylate polymer crosslinked by polymerization in the presence of allyl diglycol carbonate and a laser dye that fluoresces to produce light of a narrow wavelength (i.e., a single pure color) out the side of the optical fiber. Selection of the dye results in side light emission of the desired color ranging from blue-violet to red. The dye compositions of Naum are useful for light pipes that transmit intense monochromatic light using high concentrations of laser dyes. This approach is not suitable for providing light pipes that transmit light having substantially no color. The present invention seeks to overcome the problems of this prior art.
The present inventors have now discovered practicable polymeric light pipe compositions that counteract the carbon-hydrogen absorbance at 630 nm so that the color of the light emanating from such light pipes does not change substantially as the length of the light pipe is increased. Furthermore, the present inventors have now discovered practicable polymeric light pipe compositions that provide controlled color to the light emanating from such light pipes. This is accomplished by adding trace amounts of dyes, having particular absorbances, to polymeric light pipe core compositions to effectively counteract the carbon-hydrogen absorption at 630 nm to provide for the transmission of substantially colorless light for various lengths of light pipe when illuminated with a white light source. Additionally, it has been discovered that trace amounts of dyes having particular absorbances when added to polymeric light pipe core compositions can effectively control the color of transmitted light when illuminated with a white light source.
In a first aspect of the present invention, there is provided a composition including:
(a) one or more polymeric materials having a first light absorbance at peak maximum; and
(b) one or more dyes having a second light absorbance at peak maximum and which does not reemit light visible to the naked human eye,
wherein the first and second light absorbance at peak maximums are chosen to control the color of the light passing therethrough.
In a second aspect of the present invention, there is provided a light pipe having a core including
(a) one or more polymeric materials having a first light absorbance at peak maximum; and
(b) one or more dyes having a second light absorbance at peak maximum and which does not reemit light visible to the naked human eye,
wherein the first and second light absorbance at peak maximums are chosen to control the color of the light passing therethrough.
Terms used to describe the present invention are as follows:
The term xe2x80x9cpolymericxe2x80x9d is understood to include within its scope all types of molecules characterized as having repeating units of atoms or molecules linked to each other such as oligomers, homopolymers, co-polymers including block, random and alternating co-polymers, grafted polymers and co-polymers, terpolymers, etc.
The term xe2x80x9cANSIxe2x80x9d is understood to mean the organization called the American National Standards Institute.
The term xe2x80x9camuxe2x80x9d is understood to mean xe2x80x9catomic mass unitsxe2x80x9d which is also understood to have substantially the same numeric value as molecular weight expressed in grams per mole, or xe2x80x9cg/molxe2x80x9d.
The term xe2x80x9cb.o.m.xe2x80x9d is understood to mean xe2x80x9cbased on monomer weightxe2x80x9d.
The composition of the present invention preferably includes one or more polymeric materials having a first light absorbance at peak maximum and one or more dyes having a second light absorbance at peak maximum, wherein the first and second light absorbance at peak maximums are chosen to control the color of the light passing therethrough. The light absorbance at peak maximums may also neutralize each other so that the composition does not substantially change the color of white light passing therethrough.
Generally, polymeric materials in the composition are, preferably, of the acrylic, methacrylic, styrenic, polycarbonate, silicone, or polyglutarimide families of substantially colorless (xe2x80x9ctransparentxe2x80x9d) polymers. The polymeric materials include, more preferably, transparent plastics such as polystyrene, polycarbonate, polymethyl methacrylate, polymethyl glutarimide, and other copolymers and terpolymers of styrenes, dienes, C1-C8 n-alkylacrylates, and C1-C8 n-alkylmethacrylates. The polymeric materials which are most preferable include transparent rubbery polymers such as crosslinked polyethylacrylate and polydimethylsiloxane, and other rubbery copolymers and terpolymers of styrenes, dienes, C1-C8 n-alkylacrylates, and C1-C8 n-alyklmethacrylates which provide for compositions of the present invention having excellent flexibility.
The one or more polymeric materials according to the present invention preferably have a light absorbance at peak maximum at wavelengths between 600 and 650 nm arising from carbon-hydrogen bonds contained in the polymeric material. It is known that polydimethylsiloxane contains 81 carbon-hydrogen bonds per 1,000 amu, polyethylacrylate contains 80 carbon-hydrogen bonds per 1,000 amu, polymethyl methacrylate contains 80 carbon-hydrogen bonds per 1,000 amu, polystyrene contains 77 carbon-hydrogen bonds per 1,000 amu, and polycarbonate contains 55 carbon-hydrogen bonds per 1,000 amu. Therefore the polymeric materials have preferably greater than zero, more preferably greater than 10, and most preferably greater than 30 carbon-hydrogen bonds per 1000 atomic mass units.
Dyes are those that the United States Federal Trade Commission has classified as solvent dyes and disperse dyes. Many dyes are also classified in the Color Index by the Society of Dyers and Colourists with a C. I. Number. Dyes are known to belong to a family of dyes classified by their C. I. designation. Examples of dyes and dye families are: azo dyes (Solvent Yellow 14, Solvent Red 24, Disperse Yellow 23), quinoline dyes (Solvent Yellow 33), perinone dyes (Solvent Orange 60, Solvent Red 135, Solvent Red 179), anthraquinone dyes (Solvent Red 52, Solvent Red 111, Disperse Violet 1, Disperse Violet 26, Solvent Violet 36, Solvent Violet 13, Solvent Violet 14, Solvent Blue 56, Solvent Blue 97, Solvent Green 3, Solvent Green 28), xanthene dyes (Solvent Green 4, Solvent Orange 63), azine dyes (induline, nigrosines), methine dyes (Disperse Yellow 201), thioindigo dyes, phthalocyanine dyes, and perylene dyes.
In a preferred embodiment, when the absorbance at peak maximum of the one or more polymeric materials is at wavelengths between 600 and 650 nm, then the wavelength of the absorbance at peak maximum of the one or more dyes is between 500 and 580 nm, preferably between 520 and 570 nm, and more preferably between 535 and 560 nm in order for the one or more dyes to neutralize the green light transmitted through the one or more polymeric materials. It is known that dyes having absorbances at peak maximums at wavelengths between 500 and 580 nm range in color from orange to red to violet. Therefore the one or more dyes preferably range in color from orange to red to violet. The one or more dyes also are preferably soluble in the one or more polymeric materials.
It is also known that polymeric materials degrade compositionally in the presence of heat, oxygen, and/or visible and ultraviolet light. Such degradation causes increased absorbance of light at wavelengths less than 570 nm, causing the polymeric material to yellow. In this case, it is known that dyes having absorbances at wavelengths greater than 570 nm are useful to neutralize the absorbances at wavelengths less than 570 nm to make the light passing through the material appear less yellow. It is also known that some light sources, such as fluorescent lights, provide off-white light that appears yellow which is undesirable. It is therefore preferable that when a yellow appearance is undesirable, the one or more dyes has an absorption at peak maximum at wavelengths greater than 570 nm. Many dyes that have an absorption at peak maximum at wavelengths greater than 570 nm have a blue color.
Dyes that resist fading in the presence of visible and ultraviolet light belong to the anthraquinone, perinone, and xanthene families of dyes. Because applications and lighting systems of the present invention involve transmitting high intensity visible light, and possibly some ultraviolet light, through long path lengths of compositions of the present invention, it is preferred that the one or more dyes belong to the anthraquinone, perinone, and xanthene families of dyes. Specific examples of dyes in the anthraquinone family are listed in Table 1.
It is also known that a combination of dyes can also be used to produce a particular color shift. For example, a dye with a color corresponding to the color of the desired light output can be used in combination with an amount of red dye sufficient to neutralize the green color.
The concentration of the one or more dyes should be sufficient to provide a light absorbance at peak maximum that is preferably from 0.2% to 2000%, more preferably from 1% to 1000%, and most preferably from 5% to 100% of the light absorbance at peak maximum of the one or more polymeric materials. This percentage is referred to as the xe2x80x9cpercent ratio of the absorbances at peak maximumsxe2x80x9d, or simply xe2x80x9cfxe2x80x9d.
The amount of dye in the composition of the present invention required to produce a particular value of f is preferably calculated according to the following equation:
Cdye=0.001xc3x97fxc3x97xcex1*xc3x97xcex5xe2x88x921xe2x80x83xe2x80x83[Equation 1]
wherein Cdye is the concentration of dye in the composition expressed in units of moles dye per liter composition (xe2x80x9cmol/lxe2x80x9d); f is the percent ratio of the absorbances at peak maximums, xcex1* is the corrected absorbance at the maximum of the peak for the polymeric material expressed in units of decibels per meter (xe2x80x9cdB/mxe2x80x9d), and xcex5 is the molar absorptivity of the dye at the absorbance at peak maximum between 500 and 580 nm expressed in units liters per mole-centimeter (xe2x80x9cl/molxc2x7cmxe2x80x9d), and 0.001 is a proportionality constant.
The corrected absorbance at the maximum of the peak for the polymeric material, xcex1*, is determined from the visible absorption spectrum of the polymeric material is measured using a halogen light source and detecting the light absorbance versus wavelength spectrum using a photodiode array spectrograph. The magnitude of the absorbance at the maximum of the peak (xe2x80x9cxcex1maxxe2x80x9d) at wavelengths between 600 and 650 nm is recorded in units of decibels per meter (xe2x80x9cdB/mxe2x80x9d). The baseline absorbance (xe2x80x9cxcex1basexe2x80x9d) of the spectrum in the vicinity of xcex1max is subtracted therefrom to provide the corrected absorbance at the maximum of the peak (xe2x80x9cxcex1*xe2x80x9d). For example, in one embodiment of the present invention, xcex1max for crosslinked polyacrylate occurs at 630 nm and has a magnitude of 0.59 dB/m, xcex1base is represented by the absorbance at 600 nm which has a magnitude of 0.17 dB/m, therefore the magnitude of xcex1* is 0.42 dB/m.
The molar absorptivity of a dye at the absorbance at peak maximum between 500 and 580 nm, xcex5, is expressed in units of l/mol-cm and determined from dilute solution measurements using a suitable UV-VIS spectrophotometer. Beer""s Law provides that
Asol=xcex5xc3x97Csolxc3x97Lxe2x80x83xe2x80x83[Equation 2]
wherein Asol is the absorbance at peak maximum at wavelengths between 500 and 580 nm of the dye solution, Csol is the concentration of the dye in dilute solution expressed in units of mol/l, and L is the path length of the sample cell expressed in units of centimeters.
For example, in one embodiment of the present invention, a 0.0842xc3x9710xe2x88x923 mol/l solution of C. I. Solvent Violet 14 is prepared in ethyl acetate solvent and measured in a 1 cm path quartz sample cell using a Perkin Elmer Lambda 6 Spectrophotometer to provide a value of Asol of 1.33 dB/m at 555 nm, which provides a molar absorptivity of 15,800 l/mol-cm according to Equation 2.
According to Equation 1, a composition according to the present invention wherein the polymeric material is polyethylacrylate and the dye is C. I. Solvent Violet 14, the concentration of the dye required to provide f=50% in the composition is:
Cdye=0.001xc3x9750%xc3x970.42xc3x9715,800xe2x88x921
therefore
Cdye=1.33xc3x9710xe2x88x928 mol/l
which, is about 5 parts per billion (xe2x80x9cppbxe2x80x9d) when Cdye is multiplied by the dye molecular weight (418.5 g/mol) and divided by the composition density (1120 g/l).
The amount of dye in the composition of the present invention required to produce a particular absorbance ratio is more preferably determined according to the following method:
The spectral power distributions of a quartz tungsten halogen light source (ANSI standard projection lamp designation EKZ) is measured using a photodiode array spectrograph and the CIE (1931) x,y chromaticity coordinates are determined as described in CIE Technical Report 15.2 (2nd ed, 1986). The x, y chromaticity coordinates are similarly determined for compositions according to the present invention which vary in dye concentration. This is repeated for several different dye concentrations in a trial-and-error fashion to establish a relationship between the dye concentration and the x, y chromaticity coordinates. The optimum dye concentration is obtained when the x, y chromaticity coordinates of the dyed composition best approximates the x, y chromaticity coordinates of the light source. The expression (xcex94x2+xcex94y2)1/2 defines the color difference (xe2x80x9cxcex94x,yxe2x80x9d) wherein xcex94x is the difference in x-coordinate between the light source and the light exiting the light pipe (x-coordinate of light exiting light pipe minus the light source x-coordinate) and xcex94y is the difference in y-coordinate between the light source and the light exiting the light pipe (y-coordinate of light exiting light pipe minus light source y-coordinate). The best approximation for the optimum dye concentration occurs when the magnitude of xcex94x,y is minimized. The values of xcex94x,y calculated for compositions having a crosslinked polyethylacrylate (xe2x80x9cPEAxe2x80x9d) polymeric material and various concentrations of several anthraquinone dyes are listed in Table 2 using two different white light sources having different power spectrums. The color difference results in Table 2 predicts that the optimum dye concentration of each dye in PEA is between 2 and 6 ppb.
Polymeric materials according to the present invention may be any of those taught in the art, such as in U.S. Pat. No. 5,485,541, for light pipe or optical fiber uses, such as a polyalkyl acrylate, polymethyl methacrylate, a polyglutarimide, a silicone polymer, and the like. Suitable polymeric materials may also contain large core polymethyl methacrylate which further contains plasticizers and/or crosslinkers. Suitable polymeric materials may also be multistrand high Tg fibers, such as polymethyl methacrylate. The one or more polymeric materials are transparent, preferably flexible, and preferably processable in melt form, then later cured or crosslinked to form the final core.
High light transmission light pipe requires a cladding having a refractive index lower than that of the polymeric core. Further, the cladding needs to be deposed on the polymeric core and be able to contain the core polymer effectively. Depending on the manufacturing process, the cladding may contain the monomers which are polymerized to form the core, the core polymer only partially polymerized, the core polymer polymerized but not crosslinked, and/or the fully crosslinked core polymer. Many cladding materials are known for this purpose, especially fluoropolymers which have lower refractive indices than most of the core polymers known to the art. Preferred compositions are wherein the fluoropolymer cladding is a terpolymer of perfluoroalkyl vinyl ether/tetrafluoroethylene/hexafluoro-propylene or a terpolymer of vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene.
U.S. Pat. No. 5,485,541 also teaches many polymers other than fluoropolymers suitable for cladding of flexible light pipe or fibers. It is generally preferred for the present invention that the cladding be of lower refractive index than the core, that the cladding and the core bond at the surface in a uniform manner. It is further preferred that the cladding be readily co-extrudable with a co-extruded crosslinkable core.
Light pipes are often coupled to high-flux illuminators for conveying bright light to a desired use point (end-lit applications) or illumination or decoration utilizing the length of the light pipe (side-lit, side-emission applications). Preferred illuminators incorporate a GE Arcstream metal halide high intensity discharge lamp (or equivalent), or a quartz-tungsten-halogen (xe2x80x9cQTHxe2x80x9d or xe2x80x9chalogenxe2x80x9d) lamp, all with proper protective spectral filtering. Other useful sources include, but are not limited to, direct solar light, focused solar light, fluorescent lamps, high-, medium- and low-pressure sodium lamps, and incandescent lamps.
There are many envisioned uses of lighting systems having one or more illuminators and one or more light pipes. Lighting systems can provide for a combination of xe2x80x9cend-lightxe2x80x9d illumination and xe2x80x9cside-lightxe2x80x9d illumination. In end-light applications, light is conducted from the illumination source through the light pipe and out through the other end of the light pipe to illuminate a target. Some specific uses for end-light lighting systems include: automotive and transportation uses, such as in headlights, rear appliques, interior lighting, dashboard lights, accent lights, map readers, interior and exterior lighting of boats, trailers, campers and airplanes, and the like; retail lighting uses, such as in track lighting, display cases, point of purchase displays, and the like; emergency lighting, such as in path of egress, exit signs, pathway indicators, and the like; to indoor and outdoor commercial lighting, such as in down lights, recessed solar collectors, ground level lighting, walkway lighting, airport runway lights, architectural lighting, traffic lights, mining lights, such as hard hat lighting and mine shaft lighting; to remote source systems, such as in prison cells, hazardous environments, zoos, aquariums, art museums, and the like; residential lighting, as in novel lighting for showers, vanities; specific task lighting, such as auto mechanic lighting, surgeon/dentist lighting, xe2x80x9chigh techxe2x80x9d manufacturing lighting, endoscopes, photographic uses, and the like; signs, such as in neo-neon, edge lit signs with plastics such as Plexiglas(trademark) acrylic resins, video/electronic displays, highway signs, and the like; and, other specialty lighting, such as in toys, underwater lighting, in water fountains, pools, aquariums, bath tubs, hot tubs, deep sea diving, biological research-catalyzing culture growth, plant growth, and the like.
In side-light applications, light is conducted from one or more illumination source through the light pipe and out through the other end of the light pipe to illuminate a target. Some specific uses for side-light optical pipe include: certain automotive and transportation uses, such as in certain interior decorative lighting of boats, trailers, campers and airplanes, and the like; certain retail lighting uses, such as in signs for replacement or enhancement of neon lighting systems, where the evenness of lighting from such system is desirable, as for back-lighting of signs; for safety guidance lines in dark areas, or for under-counter and cove lighting; to remote source systems, such as in hazardous environments, zoos, aquariums, art museums; for personal safety, such as in hiking, biking, in-line skating, scuba diving, and the like; for task lighting; for entertainment and display uses, especially where the ability to change color rapidly and continuously is important, such as in amusement parks, fountains, etc.; and for architectural uses, such as alcoves, atriums, staircases, and the like.